This invention relates to semiconductor power devices and specifically to DMOS power devices. In DMOS power devices, it is necessary to provide for current sensing of the main current of the main power cells as part of the control scheme for the device. Heretofore, current sensing has been based on current mirror techniques wherein a sense current, which is a fraction of the current through the main power cell, is shunted through a sense resistor, and the voltage drop thereacross is measured to provide an indication of the main current.
In the past, a current mirror structure on a DMOS power device chip has been provided as a separate pad in an isolated section of the die. The prior art current mirror pad is not active in current production and thus is undesired overhead of the chip. Current mirrors also require an output resistor, frequently an external component, in order to convert current signals to voltage signals for sensing.
Aspects of the prior art are explained in connection with FIGS. 1 and 2. FIG. 1, a top view, represents a power MOS chip 10 with a source region 12, a gate region 14 and a current mirror 16. FIG. 2 is an expansion of FIG. 1 around the current mirror 16. The periphery region 18 is the top metal layer for the source 12 including regular active cells 20. The current mirror 16 consists of a group of active cells separated from the source cells, but having the same structure, including a metal pad 22 separated from the top metal in the periphery region 18 of the source by a dielectric gap 24. The current mirror 16 has its active cells 26 disposed within a relatively small proportion of the current mirror region, as is required by the prior art isolation techniques.
It would be desirable to eliminate the use of external components, as well as the devotion of a portion of the chip area to isolation for the current sensor, in order to improve the capacity and reliability of power MOS devices.
The following publication may be helpful in understanding some of the background of the present invention. Zommer et al., "POWER CURRENT MIRROR DEVICES AND THEIR APPLICATIONS," PCI '86 PROCEEDINGS. June 1986, pp. 275-283 describes a prior art power MOS transistor structure in general terms in connection with applications thereof.